(1) Field of the Invention
The present invention relates to a heat sink substrate having a large area and a method of manufacturing the same and, more specifically, to a large area heat sink substrate which is mounted on a power semiconductor such as a metal oxide semiconductor field effect transistor (MOSFET), IGBT and the like and on a large capacity rectifier used to an electric railcar, electric automobile and so on and a method of manufacturing the heat sink substrate.
(2) Description of the Related Art
Semiconductors have been widely used and, in particular, so-called power semiconductors including MOSFETs, IGBTs and so on which generate heat have been extensively used in various fields expanding from industrial equipment to household equipment. As the power semiconductors are applied to electric cars and automobiles including hybrid vehicles, their output power and size are outstandingly increased and the amount of heat generated by them is inevitably increased.
Power semiconductors, from which a current of several hundreds of amperes flows, are different from MPUs (microprocessor units) conventionally used in so-called personal computers and so on in the materials constituting them and the design of their structure and they may substantially generate an amount of heat of several kilowatts. Their size is, for example, about 98-375 cm.sup.2 which is at least ten times the size of the MPUs which is about 2.2-25 cm.sup.2. Thus, the power semiconductors are often used under severe conditions as to vibration, humidity, temperature, strength and so on. As a result, when they are repeatedly used many times under the very severe conditions, cracking, exfoliation and the like are caused to them and their life is ended regardless of that no cracking and exfoliation are seemingly caused thereto.
Further, the power semiconductor is required for reliability which is more severe than that required to the MPU. In particular, the power semiconductor must pass a life test of several hundreds to several thousands of times in terms of a heat cycle as a parameter, as to deformation caused by the warp and the like of a substrate mounted on it and the occurrence of cracking, in spite of that the power semiconductor has a large area. Accordingly, when a heat sink substrate having a large area of, for example, about 100-400 cm.sup.2 is warped or when a heat sink substrate, on which a plurality of semiconductor elements are mounted, generates heat and the semiconductor elements are differently expanded by the heat, cracking and exfoliation are caused to the heat sink substrate due to the warp of the heat sink substrate or the different thermal expansion of the components and straining resulting from the expansion, even if the heat sink substrate has passed a life test executed in the state that it outputs a considerably large power (10-50 W) in a severe environment. From the above-mentioned, difference in size of the heat sink substrate is an important technical problem.
It is preferable for the heat sink substrate of the power semiconductor to have thermal conductivity of at least 200 W/m.multidot.K and more preferably at least 230 W/m.multidot.K. In addition, the heat sink substrate must have suitably small thermal expansion and strength which is larger than that of a copper material. Further, more important is that the thermal conductivity of the material of the heat sink substrate is not lowered, even if heat is generated thereto, to such a degree as to injure the operation thereof when the heat sink substrate is practically used.
On the other hand, there is generally a problem that an increase in the size of a metal heat sink material makes the characteristic anisotropy thereof more outstanding. The inventors have studied and developed a single-layer composite material by mixing a copper powder with a molybdenum powder and sintering and rolling them. The single-layer composite material is considerably uniform as an entire body and has a small amount of characteristic anisotropy. The single-layer composite material has not any void as well as the thermal conductivity and thermal expansion coefficient thereof are very closely analogous to the values which are prescribed from a mixing ratio of copper and molybdenum, even if the composite material does not contain a sintering assistant agent. Thus, it is supposed that the single-layer composite material can be effectively used to a heat sink substrate for a device on which semiconductor elements are mounted.
However, when a usual plate is rolled in an ordinary process, it is economically difficult to make the characteristic anisotropy thereof to zero. When the composite material is, for example, cross rolled, the size thereof is regulated by the work rolls used in the rolling as well as it is difficult to finish the composite material without leaving straining which take places in the rolling in the interior of the composite material. Accordingly, the single-layer composite material has been not suitable as a material as a heat sink substrate which constitutes a power semiconductor having a large area and high reliability. That is, even if the joint shape of the composite material and a joint agent used to it are changed, there cannot be obtained a heat sink substrate for the power semiconductor device.
It is possible to prepare a plate member having a length of at least 200 mm in one direction and an amount of warping of 200 .mu.m. However, this plate member is inconvenient because the warping thereof is increased by residual straining while it is annealed or subjected to surface processing such as plating and the like. That is, it is required that a heat sink substrate which will be assembled to a large area power semiconductor has substantially no residual straining or a minimal possible amount of residual straining.
Further, a material that satisfies the following performances is required to a heat sink substrate used to the power semiconductor.
First, the material has thermal conductivity of at least 200 W/m.multidot.K (at a room temperature-200.degree. C.), preferably at least 230 W/m.multidot.K, and most preferably at least 300 W/m.multidot.K as a temperature increases. In this state, however, the thermal expansion coefficient of copper (=370 W/m.multidot.K), for example, is 16-17.times.10.sup.-6 /K and the Young's modulus thereof is also low (13.times.10.sup.3 kgf/mm.sup.2). In practical use, it is impossible to devise an arrangement having reliability from the material, since the heat sink substrate made of the material is cracked and exfoliated as well as the thermal expansion coefficient and Young's modulus thereof are excessively different from the elements mounted thereon and the peripheral material thereof.
To cope with the above problem, a semiconductor element is mounted on a material which is mainly composed of multilayer material, such as Cu/Al.sub.2 O.sub.3 /Cu, Cu/AlN/Cu, Cu/AlN, AlN, Al.sub.2 O.sub.3 and so on. It is essential that the heat sink substrate uses the material which has a thermal conductivity superior to that of AlN having the maximum thermal conductivity of 200 W/m.multidot.K among the materials which at least regulate a heat sink property and can be practically put into market. An Al/SiC composite material is said to be light in weight and to have high thermal conductivity and is such that when it is heated to about 120-150.degree. C., the thermal conductivity thereof is lowered by about 20%. Further, although some materials, which are put into market in the state that they are subjected to a melting and impregnating process, have thermal conductivity of 200 W/m.multidot.K at an ordinary temperature, they are not sufficiently satisfied in practical application because their thermal conductivity is lowered to 160 W/m.multidot.K at 120.degree. C.
The thermal expansion coefficient is 12.times.10.sup.-6 /K or less and preferably 9.times.10.sup.-6/ K or less. It can be said that a material which is most affected by other material in the restricting relationship with it, warping and the like is ceramic. When only this point is taken into consideration, the thermal expansion coefficient is most preferably 7-8.times.10.sup.-6 /K. However, when the thermal expansion coefficient is 9.times.10.sup.-6 /K or less, it is sufficient to take some conventional arranging methods of easing stress into consideration. Moreover, when the aforesaid semiconductor elements are mounted on a copper laminated ceramic substrate, a thermal expansion coefficient of 12.times.10.sup.-6 /K or less is acceptable.
As to Young's modulus, since the heat sink substrate is arranged as a portion of the so-called structural member of the power semiconductor device, it is important that the heat sink substrate can protect the device. The heat sink substrate of conventional power semiconductor devices can be composed of copper. Since the power semiconductor device outputs a large power, the temperature of the device increases up to about 100-150.degree. C. Thus, it is preferable that a material containing at least copper has strength which is larger than that of copper. It is found as the characteristics of pure copper that when its temperature exceeds 150.degree. C., the tensile strength thereof is outstandingly lowered. Since Young's modulus has a behavior similar to tensile strength, it is preferable that Young's modulus is at least about 15-16.times.10.sup.3 kgf/mm.sup.2.
Although it is apparent that copper cannot be employed as the material of the heat sink substrate as described above, there is an Al/SiC composite material as a commercially available heat sink material to which attention is paid recently. Since the Al/SiC composite material is light in weight and less expensive, it is examined also as a heat sink substrate for a hybrid automobile. This material has thermal conductivity of 200 W/m.multidot.K at an ordinary temperature. since the thermal conductivity lowers to about 160 W/m.multidot.K at 150.degree. C., however, the material has an essential weak point.
Recently, cooling of a large capacity rectifier is used to an electric railcar and electric automobile and becomes an important problem. Therefore, a requirement is demanded for a relatively large heat sink member on which the rectifier and relating components are mounted and which is connected to a cooler. A metal material, such as aluminum, copper, etc., is contemplated to be suitable as a material for the heat sink member from a view point of thermal conduction. These materials have a large amount of thermal expansion. However, when they are connected to a silicon unit as a main component of the rectifier and to an insulating material such as an aluminum nitride substrate or the like to which the silicon unit is assembled, there is a possibility that the materials are deformed or broken by the thermal strain resulting from the difference of thermal expansion which arises when the materials are soldered and connected to the substrate or repeatedly used. Accordingly, a material is required which has thermal expansion closely analogous to that of a semiconductor and ceramic insulating material and is excellent in thermal conduction.
When the heat sink substrate is applied to the rectifier, the heat sink substrate requires for the physical properties similar to those have been conventionally required to the heat sink substrate of a ceramic package on which semiconductor elements are mounted.
In particular, when a heat sink substrate is connected to alumina ceramic, such as a pin grid array and the like, by silver soldering, it is connected thereto at a high connecting temperature of 890-900.degree. C. In this case, the heat sink substrate is assembled under more severe temperature condition to prevent the ceramic material from being deformed or broken by thermal the strain which is caused when the heat sink substrate is cooled. Therefore, in the selection of a material to be used for the heat sink substrate, whether the material has a thermal expansion characteristic near to that of the ceramic material such as alumina, beryllia and the like is more important than whether the material has excellent thermal conduction or not. There has been proposed a composite material of tungsten (W) and copper (Cu) (hereinafter, referred to as a W-Cu composite material) and widely used as a material satisfying the above condition. A method of manufacturing the W-Cu composite material is such that an organic binder is added to and mixed with a tungsten powder, the mixture is compacted in a metal mold, heated in a reducing atmosphere such as hydrogen etc. and a powder aggregate is obtained by evaporating, decomposing and removing the organic binder. Subsequently, a porous tungsten material having prescribed porosity is obtained by sintering the powder aggregate in a reducing atmosphere and then the W--Cu composite material is obtained by infiltrating the powder aggregate with copper in a reducing atmosphere having a temperature higher than the melting point of copper.
A heat sink substrate for an IC (integrated circuit) package which uses ceramic as its constituting material must have thermal expansion near to that of alumina and beryllia to avoid the aforesaid problem of thermal strain. Thus, use is made of a W--Cu composite material infiltrated with copper in an amount of 10-15 wt %.
In the economical manufacture of the porous tungsten material infiltrated with copper in the above weight percent, a tungsten powder is added with a slight amount nickel or the like and is often used so that the porous tungsten material can be obtained at a relatively low temperature of 1200-1350.degree. C. and that the infiltration of copper can be easily executed, although thermal conduction is made lower than the most preferable value at the time.
Incidentally, in a heat sink substrate which has components relating to a large capacity rectifier mounted thereon and is connected to a cooling apparatus, such as a radiator or the like, since the heat sink substrate is connected to the rectifier (silicon unit) and to an aluminum nitride substrate on which the silicon unit is assembled by means of a low melting point material such as solder, the allowable range of the thermal expansion of the heat sink substrate is increased as compared with the case of the aforesaid ceramic semiconductor package.
Since the large capacity rectifier generates during operation, heat greatly larger than that generated by a semiconductor element. Therefore, it is an important factor in the selection of the heat sink material whether or not a material has excellent heat conduction. A large and light material is required in addition to the above factor.
Accordingly, the W--Cu composite material is used for the package on which the semiconductor element is mounted. Moreover, the W--Cu composite material is not always suitably used for the large capacity rectifier in both the characteristics and manufacturing method thereof. Further, a method of manufacturing the Cu--Mo composite material must extract physical properties which are intrinsically provided with the material as well as must be an industrially applicable method.
On the other hand, it is self-evident that a product finished by a compacting process does not have a sufficiently satisfactory outside surface condition when it is left in a honed state. The surface condition of the product in the previous Art can be enhanced and the product can be easily made by subjecting it to a rolling process in a minimum necessary range. The rolling process will be very effective if the compacting process can be intrinsically easily carried out thereby.
It is expected that the application of the present invention to a heat sink substrate with a not large but ordinary size used to a microwave package, which has been difficult to be made by the above method, can solve a problem for improving the applicability of the material.
The aforesaid heat sink has been made of a Cu--W material which is obtained by infiltrating porous tungsten with melted copper. The Cu--W material ordinarily contains copper in an amount of 10-20 wt % and has excellent characteristics that a thermal expansion coefficient is 6-7.times.10.sup.-6 /K and a thermal conductivity is 210-250 W/m.multidot.K. However, the Cu--W material has a defect that the density and weight thereof are large and which increasingly becomes an important factor to be solved as the reduction of weight and size of a part composed of it is expedited. Further, since the material is worked by being cut, it is also a problem that the thickness thereof cannot be reduced, herein a limit of thickness is 0.5 mm, and the area thereof cannot be increased, herein larger than a size corresponding to a B5 size.
Although there is commercially available a 15-20 wt % Cu--Mo material composed of porous molybdenum infiltrated with melted copper, the material has a problem in a thickness and an increase of an area likewise the Cu--W material and it cannot be said that the cost thereof is cheap.
Since a large substrate is used to a power semiconductor employed by an electric automobile and an electric rail car and generates heat in an amount larger than that generated by an ordinary semiconductor package, important characteristics required to it is a heat sink property, matching of the thermal expansion of it to other substrates and warping.
As to the size of the heat sink substrate, a substrate having a thickness of 2-4 mm and an area of 98-375 cm.sup.2 is called a large substrate. The area of the large substrate is at least ten times that of a substrate for MPU having an area of 2.2-25 cm.sup.2.
The materials called TT-RCM, registered trademark No. 2626137, are sintered body which are formed from compacted body of Cu and Mo powders and, typically, have been put into market contain copper in an amount of at least 40 wt % which material is called "RCM" as a product name of Tokyo Tungsten CO. LTD. The sintered body has a limited thickness to be rolled from the material, and which thickness depends upon the amount of copper contained in the sintered body. In particular, it is difficult to make a large substrate which is suitable for a power semiconductor in a region of the copper content of 40 wt % or less.
On the other hand, the molybdenum material impregnated with copper is referred to as "PCM". Although, the material named as "PCM20" comprises molybdenum impregnated with copper of 20 wt % and has somewhat low thermal conductivity of 170 W/m.multidot.K, it is a value which is practically applicable as a heat sink property. The thermal expansion coefficient of PCM20 is 7.times.10.sup.-6 /K which is nearer to that of silicon. As a result, PCM20 has an advantage that the matching property thereof with a substrate is improved and the quality thereof is improved because cracking, fracture and the like is not caused and degree of occurrence of warping is reduced due to increased rigidity. Whether emphasis is put on a thermal expansion coefficient or thermal conductivity depends on the application of the material, by which the number of choices can be increased.
On the other hand, although 40-60 wt % Cu--Mo materials referred to as "TT-RCMs 40-60", in particular, RCM 60 has a large thermal expansion coefficient of 12.3.times.10.sup.-6 /K, it has high thermal conductivity of 286 W/m.multidot.K. Therefore, it is widely used as a heat sink substrate for gallium arsenic, GaAs. However, since RCMs are a so-called dispersing-reinforced-type composite material in which copper particles and molybdenum particles are very finely and uniformly mixed, they have a defect that a working property is a little inferior to that of PCMs and a manufacturing cost is expensive.